The present invention pertains to alumina adsorbents for removing at least water and carbon dioxide from a fluid.
In the pre-purification of air, it is desirable to remove ambient water and CO2 prior to cryogenic distillation. Since water and CO2 form solids at liquid air temperatures, these trace air impurities must be removed to avoid plugging in the distillation system.
Alumina is typically used as a desiccant in air pre-purification systems. Alumina has several advantages in this application including (1) low cost, (2) high volumetric water capacity, (3) simultaneous water and CO2 adsorption and (4) easy description of water.
It has been found that if alumina is treated with basic salts, its CO2 capacity can be increased. U.S. Pat. Nos. 5,656,064 (Golden et al.) and 6,125,655 (Millet et al.), for example, teach applying the basic salt to the alumina by an aqueous impregnation technique. Although this technique improves the CO2 capacity of the alumina, the present inventors are aware of the following drawbacks associated with the technique. Firstly, the impregnation technique deposits salts in the pore structure of the alumina. This lowers the alumina surface area, which in turn lowers its water capacity. Secondly, the impregnation technique requires additional processing steps (e.g., impregnation followed by activation), which add to the complexity of manufacturing the material and increases its cost.
The formation of alumina/salt composite adsorbents by adding basic salts during the pelletizing process has been previously described. For example, U.S. Pat. No. 5,935,894 (Kanazirev) describes a method for preparing an alumina-based catalyst or adsorbent by contacting activated alumina powder with an aqueous solution of at least two different alkali metals, wherein at least one of the alkali metals is derived from a carboxylic acid alkali metal salt. During the activation process, the organic anion is decomposed, leaving a metal oxide. These materials are said to show improved CO2 capacity over materials produced with just one alkali metal.
In addition to their use in air pre-purification processes, composite aluminas are known to be useful as adsorbents in other separation processes.
For example, U.S. Pat. No. 5,427,995 to Ziebarth et al. discloses an alumina adsorbent for adsorbing NOx and SOx from waste gases. The adsorbent is prepared by adding an alumina stabilizing agent (e.g., silica, rare earths, titania, zirconia and alkaline earths) to precipitated alumina, forming a slurry, milling and spray drying the slurry to form stabilized spheroidal alumina particles, and impregnating the particles with an alkali metal or alkaline earth metal to form the stabilized adsorbent.
U.S. Pat. No. 5,316,998 to Lee et al. discloses an HCI adsorbent, methods of making and using the adsorbent, wherein the adsorbent comprises an activated alumina and about 5 wt % of an alkali metal oxide promoter. The alkali metal oxide is derived from a water soluble, alkali metal salt of an organic compound which decomposes at temperatures below about 500° C.
U.S. Pat. No. 5,096,871 to Lever et al. discloses alumina-containing acid adsorbents for removing acidic materials from fluids. The adsorbents comprise activated alumina and an amorphous alkali aluminum silicate. The adsorbents are prepared by a process comprising treating activated alumina with a (preferably aqueous) solution of an alkali metal silicate followed by an alkali metal aluminate, forming agglomerates of the treated alumina at a temperature below about 90° C., aging the agglomerates at a temperature in the range of 20-90° C., and calcining the aged agglomerates at a temperature in the range of 200-500° C.
U.S. Pat. No. 4,855,276 to Osborne et al. discloses adsorbent compositions for removing compounds from gases, wherein the adsorbent compositions comprise alumina, carbon and in certain embodiments, sodium bicarbonate and impregnates such as Group 1A metal hydroxides and Group 7A salts of Group 1A metals. The compositions are prepared by a process comprising the steps of forming a mixture of activated alumina, carbon and water into a cohesive unit, and curing the cohesive unit at an elevated temperature.
U.S. Pat. No. 4,755,499 to Neal et al. discloses adsorbents comprising (a) an alumina substrate having a pore volume between 0.4 and 0.8 cc/g, and (b) an alkali or alkaline earth component, for example, sodium carbonate, wherein the amount of the alkali or alkaline earth component is between 50 and 400 μg/m2 of the substrate. The adsorbents are said to be outstandingly effective for the removal of nitrogen oxides, sulfur oxides and hydrogen sulfide from waste gas streams. The patent teaches loading the alkali or alkaline earth component onto the alumina substrate by spraying the substrate with a solution of a salt of the alkali or alkaline earth component.
Despite the foregoing developments, it is desired to provide an improved process for removing water and/or CO2 from a gas stream. It is further desired to provide an improved adsorbent and an improved apparatus for use in such a process.
All references cited herein are incorporated herein by reference in their entireties.